The invention relates to an electromagnetic interference suppression filter for use by mounting on printed circuit boards or the like of various electric circuits.
In the electronic apparatus, an electromagnetic interference suppression filter is installed intermediately in the electronic circuit in order to obviate the occurrence of malfunction due to noises induced by the power source line or the signal line and to prevent the noise originating in the apparatus from affecting other equipment.
In conventional electromagnetic interference suppression filters, such as that shown in FIG. 1, two internal electrodes 2a, 2b are provided on the inner periphery of a tubular dielectric 1, an external electrode 3 being provided on the outer periphery of the dielectric 1 so as to be opposed to the two internal electrodes 2a, 2b, a penetration shaft provided with a ferrite bead 4 being fitted through the tubular dielectric 1 so that the ferrite bead 4 may be located inside the dielectric 1, grommets 5 being fixed to the internal electrodes 2a, 2b respectively, the grommets 5 being conducted to the penetration shaft 6, a metal fixer 7 being secured to the external electrode 3 by solder, thereby resulting in an LC.pi. type filter circuit by means of inductance formed by the ferrite bead 4, capacitance C.sub.1 formed between the internal electrode 2a and the external electrode 3, and capacitance C.sub.2 formed between the internal electrode 2b and the external electrode 3, as shown in FIG. 2.
When the conventional electromagnetic interference suppression filter was installed on a printed circuit board, the following process was required. The printed circuit board was provided with a hole for receiving the electromagnetic interference suppression filter, the metallic fixer 7 being soldered to the ground part of the printed circuit board after the electromagnetic interference suppression filter was directly fitted into said hole, and the penetration shaft 6 then being bent at one end thereof or a lead wire or the like being fixed thereto. The lead wire was then soldered to a predetermined part of the printed circuit board. Alternatively, as shown in FIG. 3, the electromagnetic interference suppression filter 9 was fitted into one of a plurality of holes provided on a metallic shield plate 8, the metallic fixer 7 being soldered to the shield plate 8, the shield plate 8 being soldered to the grounded part of the printed circuit board, and the penetration shaft also being soldered to a predetermined part of said printed circuit board. Thus the mounting operation was very complicated and time-consuming, and, particularly when a plurality of electromagnetic interference suppression filters were required, the complicated mounting operation was a bottleneck in production.
In the conventional electromagnetic interference suppression filter, the ferrite bead 4 was fitted inside the dielectric 1. Thus, not only did the dielectric have to have a large inside diameter but also brush coating or similar inefficient methods had to be employed in order to form the internal electrodes 2a, 2b inside the dielectric 1, and the internal electrodes 2a, 2b had to be electrically connected to the penetration shaft 6 by use of the grommets 5, 5, respectively. This complicated construction caused another bottleneck in mass production.